The present invention relates to an improved electromagnetic spring-wound clutch which may be advantageously employed to connect an air conditioning compressor to an engine of an automotive vehicle. This invention constitutes a novel and advantageous improvement to an ELECTROMAGNETIC SPRING-WOUND CLUTCH disclosed in copending U.S. patent application Ser. Nos. 866,645, filed Jan. 3, 1978 and 917,316, filed June 19, 1978, which are assigned to the same assignee as this application.
An electromagnetic spring-wound clutch of the general type to which the present invention relates is disclosed in U.S. Pat. No. 3,735,847 and comprises coaxial input and output members. A clutch disc is rotatably provided on the input member. Furthermore, a coil-wound spring is wound around the input and output members and connected at its opposite ends to the clutch disc and output member. An electromagnetic coil attracts the clutch disc into frictional engagement with the input member, causing the coil-wound spring to be tightly wound around the input and output members and drivably connect the input member to the output member. When the coil is de-energized, the clutch disc disengages from the input member, and the coil-wound spring releases the input member for rotation relative to the output member.
The clutch further comprises a generally tubular cover which protectively surrounds the coil-wound spring. With the coil de-energized, the coil-wound spring radially expands due to its own force into engagement with the inner surface of the cover. Furthermore, a spring means urges the clutch disc away from the input member and into engagement with the cover when the coil is de-energized.
Although the spring-wound clutch as disclosed in the prior art is generally effective and advantageous, it suffers from several problems which have heretofore remained unsolved. As a practical design consideration, an axial gap must be provided between the input and output members. The coil-wound spring, as it winds around the input and output members to engage the clutch, tends to partially enter the gap between the input and output members. As a result, the edges of the coil-wound spring scratch against the end faces of the input and output members causing wear and rough engagement of the clutch. Eventually, the impact load at clutch engagement increases to an unacceptable extent, resulting in extremely abrupt engagement.
In order to prevent excessive impact load and unpleasant noise at engagement, a certain amount of slippage of the output member relative to the input member must occur. However, in the spring-wound clutch developed thus far, the coil-wound spring tends to tightly embrace the input and output members in an almost instantaneous manner when the coil is energized. Generally, relative slippage should occur for 40-60 ms after energization of the coil to avoid undesirable abrupt vehicle speed changes. To provide 50 ms of slippage at 3000 rpm, a relative slippage of about 2.5 revolutions is required. However, it has been impossible to provide such an optimum amount of slippage in the electromagnetic spring-wound clutch at its state of development heretofore.